At the time of his death in April last year, Pol Pot was reported to have suffered a heart attack.

The Hong Kong-based Far Eastern Economic Review says the United States turned down an opportunity to arrest Pol Pot, under whose regime an estimated 1.7 million Cambodians died.

Thayer: 'There is no court, no indictment, no arrest warrant, for any Kmer Rouge leader'

The Review's Cambodia Correspondent, Nate Thayer, told the BBC many very senior Khmer Rouge sources, who have now defected to the government, had spoken in great detail about the circumstances of Pol Pot's death.

They said their leader took a cocktail of anti-malarial pills and tranquillisers on 15 April to avoid being taken alive.

But the US State Department has denied there was ever an offer for Pol Pot to be handed over.

US 'unprepared despite rhetoric about justice' says Thayer

"The suggestion that we were engaged in a negotiation where such an offer was made and we rejected it is completely counter to the information provided to me," said the State Department spokesman, James Rubin.

He said the US had done an enormous amount to try to assist in the bringing to justice of Khmer Rouge leaders responsible for war crimes.

Mr Thayer says he is certain Pol Pot took his own life and he offers the following chain of events leading up to the suicide:

• Khmer Rouge military commander Ta Mok offers to turn Pol Pot over to the US on 25 March 1998.

• Lacking an indictment, an arrest warrant or a court in which to try the aged rebel leader, the Americans refuse.

• Furious behind-the-scenes effort in Washington begins to establish grounds for an arrest and find a country willing to hold Pol Pot while a trial could be arranged.

• Before the process could be completed, Pol Pot takes his own life.

During its heyday in the 1970s, the Khmer Rouge instituted a bloody regime of terror.